1. The Field of the Invention
The present invention relates generally to continuously varying automatic transmissions in vehicles. More particularly, it concerns a vehicle transmission system which continuously and variably divides input torque between at least two torque paths to thereby maintain an optimal gear ratio, without the aid of brakes, clutches, belts, external motors or any other secondary input force.
2. The Background Art
Transmission devices are well known in the field of motorized vehicles for converting and transmitting the power generated by an engine to an output shaft. It is known to make efficient use of the motor power by adjusting the gear ratio between the input and output shafts responsive to variation in loads on the output shaft and/or the input shaft. For example, a higher gear ratio often provides a more efficient transfer of power for larger loads on the output shaft. Conversely, a lower gear ratio often suffices for lesser loads on the output shaft.
The need to optimize the motor power by varying the gear ratio has motivated the development of many different transmission devices capable of shifting between gear ratios. The standard transmission is known in the field to permit an operator to shift between various gear ratios by operating a hand-shift lever in conjunction with a foot clutch. Automatic transmissions have been developed which accomplish an automatic shifting between gear ratios responsive to increases and decreases in load on the output shaft.
Another approach to vehicle transmissions is the concept of a continuously variable transmission, which automakers have attempted for years to develop. It is thought that energy transfer between the motor and the output shaft can be further optimized by providing an infinite range of gear ratios between the minimum and maximum gear ratios, instead of merely four or five different gear ratios. This idea is a good one, since variation in load on the output shaft tends to increase/decrease gradually and continuously. Indeed, it stands to reason that provision of an optimal gear ratio for each and every load variation on the output shaft would maximize the efficiency of the vehicle. However, there have been many practical difficulties with the prior art transmission devices which offer continuously variable gear ratios.
For example, U.S. Pat. Nos. 5,106,353 (issued on Apr. 21, 1992 to Ra et al.) 4,625,588 (issued on Dec. 2, 1986 to Brickley), and 3,429,200 (issued on Feb. 25, 1969 to Green) disclose continuously varying transmission devices. These and other prior art continuously varying transmission devices known to applicant are characterized by a number of disadvantages. They require a brake, clutch, torque convertor or some other secondary input force necessary to permit continuous variation between gear ratios. They also require a large number of gears and support components, and are therefore cumbersome in manufacture, maintenance and operation.
The above-mentioned prior art variable-ratio transmission devices typically involve two differential gear sets which divide the input torque into two torque components. The two torque components are eventually linked back together to produce a resultant output torque. The variation in gear ratio is accomplished by either (i) restricting a key gear member to unidirectional rotation to force a particular proportional division of torque between the two torque components, or (ii) accelerating or decelerating a key gear member with a clutch or brake to force the input torque to be gradually divided between the two torque components. The division of the input torque between the two separate differential gear sets results in a continuous variation in the overall gear ratio. However, the secondary input force required to effectuate variation in gear ratio, whether in the form of a brake, clutch, torque convertor or other input force, necessarily results in significant power loss. It is ironic that significant power loss is required for operation of these prior art continuously variable transmissions, when the purpose of continuous gear-ratio variation is to conserve and optimize power output.
There is thus a need for an automatic, continuously variable transmission which is simple in design, manufacture and operation. There is a further need for such a transmission which does not require the aid of brakes, clutches, torque convertors, belts, external motors or any other secondary input force, to accomplish continuous variation between an infinite range of gear ratios.